1. Field of the Invention
The present invention relates to a capacitive sensing device capable of measuring various dynamical forces.
2. Description of the Related Art
A capacitive sensing device with a diaphragm is known as a device for measuring various dynamical forces. The device comprises a silicon substrate and a glass substrate. The diaphragm is formed by etching the silicon substrate with micromachining. The diaphragm can be deformed according to the various dynamical forces to be measured, such as pressure or acceleration. The glass substrate is disposed close to and separated by a small gap from the silicon substrate provided with the diaphragm so as to form a sensing capacitor. When the dynamical force is applied to the diaphragm, the diaphragm is deformed and a capacitance of the sensing capacitor varies. Accordingly, the dynamical force can be estimated through the measurement of the capacitance of the sensing capacitor. The sensing capacitor is inserted in an oscillation circuit to determine a frequency of an oscillation pulse signal. The capacitance of the sensing capacitor is measured by a frequency value of the oscillation pulse signal. That is, the capacitance is converted into the frequency value by a capacitance-frequency converter (c-f converter) and the frequency value of the oscillatory pulse signal output from the c-f converter is measured by a counter.
This known capacitive sensing device, however, has problems that the silicon substrate and the glass substrate joined to the silicon substrate are strained due to differences in thermal expansion coefficient between silicon and glass, and as a result, an output signal level of a CMOS circuit which is included in the c-f converter drifts with a temperature variation. Consequently, the frequency of the oscillatory output of the c-f converter varies according to temperature (temperature drift of the zero point) even if the dynamical force remains constant as a reference force, and a rate of a frequency deviation to a deviation of the applied dynamical force varies with a temperature variation (temperature drift of the sensitivity). Thus there occurs measuring errors in the measurements of the dynamical force.
A temperature compensation method disclosed in Japanese Patent Laid-open (Kokai) No. Hei 2-130014 employs an oscillation circuit where a driving voltage is designed so as to minimize the frequency shift for the temperature variation. FIG. 5 shows a capacitance detecting circuit employing a Schmidt trigger circuit and capable of temperature compensation. The temperature drift of frequency of the output of this capacitance detecting circuit can be reduced to a minimum when it is driven by a suitable driving voltage V.sub.DD. FIG. 6 shows a relation between the frequency of the output signal and the driving voltage for different temperatures. Although the foregoing known capacitive sensing device is capable of temperature compensation of an offset value, i.e., a zero point, of a detection value, it is incapable of temperature compensation of the sensitivity. Even in the temperature compensation of the zero point, a curve representing the compensated zero point temperature characteristic, i.e., a curve representing a rate of the frequency deviation of the output signal for a temperature when no pressure is applied to the capacitive sensor, is flat only in a narrow temperature range, so that the temperature compensation of the frequency of the output is possible only in a narrow temperature range.